1. Field of the Invention
The present invention relates to passenger aircraft video distribution systems, and more particularly concerns control of RF signal levels at different frequencies of such a system.
2. Description of Related Art
Each passenger of a passenger aircraft may be provided with an individually controllable electronics box unit to enable personal selection from among a group of different audio signals and a group of different video signals. The audio signals, and also the video signals, together with their own audio, are transmitted as entertainment signals to each of the passenger seats from one or more central audio and/or video sources. The various entertainment signals are modulated upon individual ones of a plurality of RF carriers of different frequencies and transmitted to the individual passenger seats via a series of transmission stations which are interconnected by lengths of coaxial cable. The stations are remotely positioned from one another so that the total length of cable between the video source and the passenger seat tuners may be in the order of one hundred to several hundred feet. At the various transmission stations the several signals are split into different groups for transmission to different areas of the aircraft, and signals are tapped for use at each individual passenger seat. It is essential in such a system to ensure that optimum RF input levels are provided to the video tuners at the individual passenger seat units. If the RF level at any seat unit is too low, the signal is weak, and video may be poor, exhibiting "snow". At least partly because each video signal from a number of video channels is modulated upon its own RF carrier, if the RF signal levels are too high, excessive intermodulation products may be generated which would be visible to the passengers using the video tuners.
In a co-pending application for Automatic RF Leveling in Passenger Aircraft Video Distribution System, filed Apr. 8, 1991, Ser. No. 07/681,850 and assigned to the assignee of the present application, there is described in detail a system for automatically controlling RF signal levels of the RF video carriers. However, control of RF signal level alone, equally over all carrier frequencies, is not sufficient in view of the relatively great lengths of cable through which the signal is transmitted from the video source to the passenger seat. Radio frequency signals in various frequency ranges, such as in the frequency range of between about 50 and 300 megahertz, are subject to frequency dependent losses (e.g. cable losses) in transmission of such signals through standard coaxial cable. Losses at higher frequencies are considerably greater than losses at lower frequencies. Accordingly, a plot of radio frequency or RF signal amplitude versus frequency at the output of a long cable exhibits a "tilt", showing a relatively higher amplitude at lower frequencies and a lower amplitude at higher frequencies because of the greater cable loss at the higher frequencies. Therefore, in order to obtain proper signal levels over all video channels, it is also necessary to provide proper equalization for the signals being fed to the passenger seat tuner to ensure proper RF levels at both high and low frequency channels and those in between.
One possible approach to handling this problem of ensuring proper "tilt" of the RF signal is to provide a fixed equalizer at some station upstream from the seats. For a system of a predetermined and fixed configuration such an approach may be satisfactory. However, this is not practical in the environment of a passenger aircraft because the aircraft configuration is frequently changed. Airlines often add or remove seats from one row or column, add additional channel capability by including additional sources of video channels at other frequencies, or otherwise reconfigure the entertainment distribution system. When such reconfiguration occurs and cable length is increased, video signals encounter additional cable loss, thereby changing the tilt at the end of the lengthened cable. Accordingly, if the aircraft system is reconfigured, the fixed equalizer must be changed to compensate for changed cable losses caused by changed cable length.
Another situation in which a fixed equalizer may be inadequate is the occurrence of a failure of an amplifier or other component. For example, if an amplifier in a system degrades in such a way that its output adds some unexpected tilt to the system, that is, should the amplifier degrade in such a way that it will amplify some frequencies more than others, it may be necessary to compensate by varying equalization to compensate for the changed tilt caused by the failed amplifier. The various electronic stations, which contain gain control amplifiers, are ordinarily not easily accessible, nor is there often an available technician who is sufficiently knowledgeable for adjusting and readjusting or replacement of equalizers in the system. Further, it is also desirable to report the nature and location of any failure or degradation of signals to a central location for diagnostic purposes. Present systems provide for no such diagnosis.
Accordingly, it is an object of the present invention to provide a multiple signal distribution system that avoids or minimizes above mentioned problems.